B Surface couplings Sarcoplasmic reticulum 0.5 mµ0.2 mµ 0.5 mµ •Fig. 14.3 Junctions,membranes,andmyofilaments,insmoothmuscle.A,TransmissionelectronmicrographofGapjunctionbetweenintestinalsmoothmusclecells.B,Longitudinalviewofapulmonaryarterysmoothmusclecell.Thesarcoplasmicreticulumisstainedwithosmiumferricyanideandappearstoformacontinuousnetworkthroughoutthecellconsistingoftubules,fenestratedsheets(long arrows), andsurfacecouplingsatthecellmembrane(short arrows). C,Transversesectionofabundleofvenoussmoothmusclecellsillustratingtheregularspacingofthickfilaments(long line) andtherelativelylargenumberofsurroundingthin(actin)filaments(inset). Densebodies(arrowheads) aresitesofattachmentforthethinactinfilamentsandequivalenttotheZlinesofstriatedmuscles.Elementsofsarcoplasmicreticulum(short line) occurattheperipheryofthesecells.D,Scanningelectronmicrographoftheinnersurfaceofthesarcolemmaofanintestinalsmoothmusclecell.Longitudinalrowsofcaveolaeprojectintothemyoplasm(small, light-colored spheres), surroundedbydarkerelementsofthetubularsarcoplasmicreticulum.Theattachmentsofthinfilamentstothesarcolemmabetweentherowsofmembraneelementswereremovedduringpreparationofthespecimen.(AandD,FromMottaPM[ed].Ultrastructure of Smooth Muscle. Norwell,MA:KluwerAcademic;1990.BandC,FromSomlyoAP,SomlyoAV.Smoothmusclestructureandfunction.In:FozzardHAetal.[eds].The Heart and Cardiovascular System.2nded.NewYork:RavenPress;1992.) muscle cells. The adherens junction appears as thickened regions of opposing cell membranes that are separated by a small gap (≈60 nm) containing dense granular material. Thin filaments extend into the adherens junction to allow the contractile force generated in one smooth muscle cell to be transmitted to adjacent smooth muscle cells.
Embryonic smooth muscle cells do not fuse, and each differentiated cell has a single centrally located nucleus. Though dwarfed by skeletal muscle cells, smooth muscle cells are nevertheless quite large (typically 40–600 µm long). These cells are 2 to 10 µm in diameter in the region of the nucleus, and most taper toward their ends. Contracting cells become quite distorted as a result of the force exerted on the cell by attachments to other cells or to the extracellular matrix, and cross sections of these cells are often very irregular.
Smooth muscle cells lack T tubules, the invaginations of the skeletal muscle sarcolemma that provide electrical links to the sarcoplasmic reticulum (SR). However, the sarcolemma of smooth muscle has longitudinal rows of tiny sac-like inpocketings called caveolae (see